Polishing apparatus and method for detecting foreign matter on polishing surface

ABSTRACT

A polishing apparatus comprises a polishing tool having a polishing surface, and a holder device (top ring) for holding a semiconductor wafer (a substrate). The polishing apparatus further comprises a color CCD camera for taking a color image of a region on the polishing surface; an image processor for determining whether or not any foreign matter exists on the polishing surface based on a condition of a color in color image data acquired by the color CCD camera; and an apparatus operation control section which in response to determination of the image processing section, stops relative movement between the semiconductor wafer and the polishing surface and separates the top ring and the polishing surface from each other.

FIELD OF THE INVENTION

The present invention generally relates to a polishing apparatus, andmore particularly, to a polishing apparatus capable of detecting foreignmatter on a polishing surface, which might be produced especially by aslip-out event of a substrate during a polishing process, and also to amethod for detecting foreign matter on a polishing surface.

DESCRIPTION OF THE PRIOR ART

Conventionally, a polishing apparatus has been commonly employed as anapparatus for producing a planar surface of a semiconductor substrate.This type of polishing apparatus has a configuration in which asubstrate held by a top ring (a substrate holder device) is pressedagainst a polishing surface of a polishing pad mounted on a top surfaceof a turntable, and then the substrate and the polishing surface areslidably moved relative to each other while supplying a slurrycontaining abrasive grains onto the polishing surface, thus to polish asurface, to be polished, of the substrate.

In the polishing apparatus having the above configuration, sometimes ithappens that the substrate slips or jumps out from the top ring duringpolishing. In such an event, if a polishing operation is continuedwithout taking an appropriate remedying action, not only would thisslipped-out substrate break, but also the polishing apparatus may bedamaged. More disadvantageously, in case of breakage of the substrate,removal of broken pieces of the substrate and re-conditioning of thepolishing pad may be required before restarting a polishing process;leading to a significantly low rate of productivity.

In order to deal with such a situation, a camera is used for taking animage of the polishing surface of the polishing pad, and acquired imagedata is processed by which it is possible to detect a slip-out event ofthe substrate, or existence of any foreign matter on the polishing pad,so that if either of these two conditions is detected, a polishingprocess may be suspended.

However, a conventionally available camera employed for theabove-mentioned purpose is a monochrome camera, with which detection offoreign matter cannot be ensured in a case, for example, when a color ofthe polishing surface and a color of the foreign matter are differentfrom each other but both have similar brightness, thus resembling eachother in tone and lacking in contrast. Especially for a polishing padhaving a polishing surface of a dark color (e.g., a black polishingsurface), it has been difficult to detect a slipped out substrate thatmight be a semiconductor substrate.

SUMMARY OF THE INVENTION

The present invention has been made in light of problems as pointed outabove, and an object thereof is to provide a polishing apparatus capableof detecting an existence of any foreign matter on a polishing surfacein a more reliable manner, and a method thereof.

In order to solve the above problems, according to an aspect of thepresent invention, a polishing apparatus comprising a polishing surfaceand a substrate holder device for holding a substrate and pressing thesubstrate against the polishing surface, with a surface to be polishedof the substrate brought into contact with the polishing surface, inwhich the substrate can be polished by a relative movement between thesubstrate and the polishing surface, is provided. The polishingapparatus further comprises a color camera for taking an image of aregion on the polishing surface defined in the vicinity of the substrateholder device, and an image processing section for determining whetheror not foreign matter exists on the polishing surface based on acondition of a color in a set of image data acquired by the colorcamera.

According to another aspect of the present invention, a polishingapparatus characterized in that an image processing section comprises:an identifying device for identifying whether or not a color of eachpoint in image data represents a color of foreign matter; and adetermination device which determines that foreign matter exists if atotal area of those points having colors screened and identified torepresent the foreign matter is larger than a predetermined thresholdvalue, is provided.

According to another aspect of the present invention, a polishingapparatus characterized in that an image processing section comprises: ascreening device for screening to identify whether or not a color ofeach point in a set of image data is identical with a color of foreignmatter, which has been previously stored as a reference color, oridentical with a color representative of a polishing surface; and adetermination device which determines that foreign matter exists ifeither an area corresponding to the reference color or an area notcorresponding to the reference color goes beyond a correspondingpredetermined threshold value, is provided.

According to still another aspect of the present invention, a polishingapparatus is provided, which is characterized in further comprising anapparatus operation control section, which in response to determinationof the image processing section that foreign matter exists, stopsrelative movement between a substrate and the polishing surface andseparates the substrate holder device and the polishing surface fromeach other.

According to another aspect of the present invention, a detection methodis provided, for detecting foreign matter on a polishing surface duringa polishing process where a substrate is being polished by relativemovement between the substrate and the polishing surface while pressingthe substrate against the polishing surface, with the method comprisingsteps of: taking an image of a predetermined region on the polishingsurface by using a color camera; screening to identify whether or not acolor of each point in a set of image data taken by the color camera isidentical with a color of foreign matter, which has been previouslystored as a reference color, or identical with a color representative ofthe polishing surface; and determining that foreign matter exists ifeither an area corresponding to the reference color, or an area notcorresponding to the reference color, goes beyond a correspondingpredetermined threshold value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic front view illustrating main components of apolishing apparatus.

FIG. 2 is a general flow diagram showing an example of a foreign matterdetection method.

FIG. 3( a) and FIG. 3( b) show polishing surfaces of a polishing tool,respectively, represented in a two-color pattern.

FIG. 4 is a schematic diagram illustrating a specific method fordetecting foreign matter on a polishing surface.

FIG. 5 is a general schematic view illustrating an example ofconfiguration of a polishing apparatus equipped with a cleaning unit.

In the drawings, reference numeral 110 (110 a, 110 b) designates apolishing apparatus, 1 a polishing tool, 2 a turntable (polishingtable), 3 a table turning shaft, 4 a top ring (substrate holder device),5 a top ring turning shaft, 6 a top ring swing arm, 7 a swing armturning shaft, 10 a color CCD camera (a color camera), 40 an imageprocessing section, 45 an apparatus operation control section, 50 anabrasive liquid supply pipe, symbol “S” an abrasive liquid (a slurry)and “W” a semiconductor wafer (a substrate), respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An exemplary chemical and mechanical polishing plant (CMP plant)equipped with a polishing apparatus and a cleaning unit will now bedescribed, which represents one application of the present invention.

FIG. 5 is a general schematic view illustrating an example of apolishing plant of this type. Polishing plant 110 comprises, as shown inFIG. 5, a pair of similarly configured polishing apparatuses 110 a, 110b which are located symmetrically on left and right sides of the plant.A cleaning unit 126 includes a pair of primary cleaning machines 126 a1, 126 a 2, a pair of secondary cleaning machines 126 b 1, 126 b 2, anda pair of turn-over machines 128 a 1, 128 a 2, respectively, which arelocated symmetrically on the left and the right sides corresponding torespective polishing apparatuses 110 a, 110 b, and further includes twotransfer equipments 124 a, 124 b. In addition, two load and unloadstations 122, 122 are arranged symmetrically on the left and the rightsides, respectively.

The polishing apparatus 110 a, 110 b comprises a turntable (a polishingtable) 2 a, 2 b and a top ring 4 a, 4 b for pressing a semiconductorwafer held on an under surface thereof against the turntable 2 a, 2 b soas to polish the wafer.

In the polishing apparatus having such a configuration as describedabove, a semiconductor wafer is transferred by the transfer equipments124 a, 124 b from load and unload station 122 to a delivery table 138 a(or 138 b) to be sucked there onto the under surface of the top ring 4 a(or 4 b), which is in turn moved to a position above the turntable 2 a(or 2 b). A polishing tool 1 a, 1 b, such as a polishing pad or a bondedabrasive having a polishing surface formed on a top surface thereof, ismounted on a top of the turntable 2 a, 2 b. Then, while supplying aspecified abrasive liquid (in specific, for polishing an insulation film(an oxide film) on a silicon wafer, the abrasive liquid of alkalineaqueous solution with abrasive grain particles having a specifiedparticle size suspended therein), and also while rotating the turntable2 a (or 2 b) and the top ring 4 a (or 4 b) respectively, thesemiconductor wafer is pressed against the polishing surface thus topolish the semiconductor wafer. After having been finished with thispolishing process, the semiconductor wafer is passed through a cleaningand a drying process and delivered back to the load and unload station122.

The primary cleaning machine 126 a 1, 126 a 2 is a low speed rotary typecleaning machine in which a plurality of vertical rollers 130 isarranged surrounding the wafer, and the wafer is held at an outerperiphery thereof by grooves formed on upper peripheries of the rollers130 so that the wafer may be driven to rotate by rotation of the rollers130, wherein cleaning members made of sponge in the form of a roller ora pencil are provided so as to come into contact with and to beretracted from the wafer from above and below directions. The secondarycleaning machine 126 b 1, 126 b 2 is a high speed rotary type cleaningmachine having a wafer gripping arm extending radially from a top end ofa turning shaft.

After the above-discussed polishing process, a cleaning process isperformed in the following manner. First, in the primary cleaningmachine 126 a 1 (or 126 a 2), the wafer is subjected to scrub cleaningin which the wafer is scrubbed with cleaning polishing members to becleaned while being rotated, and also supplied with a cleaning liquidover top and bottom surfaces thereof.

Then, in the secondary cleaning machine 126 b 1 (or 126 b 2), the waferis further cleaned and subsequently subjected to a drying process by ahigh speed spinning of the secondary cleaning machine. After havingfinished with these cleaning and drying processes, the wafer is returnedto the load and unload station 122 by a clean hand of the transferequipment 124 b.

Two different types of operation may be performed selectively in theabove described facility of polishing apparatus: one is a paralleloperation in which two polishing apparatuses 110 a, 110 b provide apolishing process independently from each other for wafers suppliedthereto respectively, and the other is a serial operation in which asingle wafer is transferred through the two polishing apparatuses 110 a,110 b sequentially so as to be subjected to different polishingprocesses therein respectively.

In the parallel operation, in which each of the polishing apparatuses110 a, 110 b functions independently for providing both a regularpolishing and a finishing polishing, a water polishing operation inwhich only water is supplied rather than an abrasive liquid may beperformed at different timings between the respective polishingapparatuses 110 a, 110 b so that the transfer equipments 124 a, 124 bmay transfer the semiconductor wafer in an efficient manner.

Since this polishing facility comprises two polishing apparatuses 110 aand 110 b as well as the primary and the secondary cleaning machines 126a 1, 126 a 2, 126 b 1 and 126 b 2, two separate wafer processing linesmay be established: one is a first wafer processing line providingsequential steps comprising a polishing process by using the polishingapparatus 110 a, a primary cleaning process by using the primarycleaning machine 126 a 1 and a secondary cleaning process by using thesecondary cleaning machine 126 b 1, and the other is a second waferprocessing line providing sequential steps comprising a polishingprocess by using the polishing apparatus 110 b, a primary cleaningprocess by using the primary cleaning machine 126 a 2 and a secondarycleaning process by using the secondary cleaning machine 126 b 2, andtherefore these semiconductor wafer transfer lines can be operatedindependently without interfering with each other, thus improvingefficiency of a cleaning operation.

In the serial operation, after regular polishing has been applied to asemiconductor wafer by polishing apparatus 110 a, the semiconductorwafer is transferred to the polishing apparatus 110 b, where a waterpolishing operation is applied to the wafer. If there is no problem ofcontamination on the polishing apparatus, the semiconductor wafer may betransferred directly from polishing apparatus 110 a to polishingapparatus 110 b by transfer equipment 124 a. If there is a problem ofcontamination, after the regular polishing having been applied to thesemiconductor wafer in the polishing apparatus 110 a, the semiconductorwafer should be transferred to primary cleaning machine 126 a 1 bytransfer equipment 124 a to be cleaned therein, and subsequentlytransferred to polishing apparatus 110 b, where in turn the waterpolishing operation is applied to the semiconductor wafer.

In this case, any preferred chemicals may be selected depending on atype of slurry used in polishing apparatus 110 a, so as to be addedduring a cleaning process in primary cleaning machine 126 a 1. In thisserial operation, since the regular polishing operation and the waterpolishing operation are independently performed on separate turntables 2a, 2 b, respectively, changing the polishing liquid to be supplied ontothe turntable from an abrasive liquid to purified water and vice versa,is not necessary, thus preventing increase of loss time in operation aswell as increase of consumption of the abrasive liquid and purifiedwater.

The present invention further comprises a foreign matter detectiondevice arranged in the above-described polishing apparatus 110 (110 a,110 b), for detecting whether or not foreign matter exists on thepolishing surface due to a slip-out of a semiconductor wafer (asubstrate) while being polished.

FIG. 1 is a schematic front view illustrating main components ofpolishing apparatus 110. As shown in FIG. 1, polishing apparatus 110comprises a turntable (a polishing table) 2, a top ring (a substrateholder device) 4, a color CCD camera 10, an image processing section 40for processing a set of image data acquired by the camera, and anapparatus operation control section 45 for controlling an overalloperation of the polishing apparatus 110. Each of these components willbe described below.

The turntable 2 is of a disc-like shape and has a table turning shaft 3mounted in a central location on a lower surface thereof, and furtherhas a turntable driving section 15 below the table turning shaft 3,which drives the turntable 2 to rotate via the table turning shaft 3. Apolishing tool 1 formed by, for example, a polishing pad or a bondedabrasive (abrasive grains bonded by using a resin binder) is mounted ona top surface of the turntable 2.

The top ring 4 has a top ring turning shaft 5 mounted in a centrallocation on a top surface thereof with an upper portion of the top ringturning shaft 5 inserted into a top ring swing arm 6, so that the topring 4 may be driven to rotate and/or to move up and down by a top ringrotary driving device 61 and a top ring vertical driving device 63, eacharranged on the top ring swing arm 6. The top ring swing arm 6 isdesigned to be swung by a swing arm turning shaft 7. This means that thetop ring 4 is operatively designed so as to move freely between adelivery table 138 (a, b) and the turntable 2 (a, b) shown in FIG. 5with aid of the swing arm turning shaft 7. In addition, an abrasiveliquid supply pipe 50 for supplying an abrasive liquid (a slurry) S isarranged above the turntable 2.

The color CCD camera 10 is attached on a side wall of the top ring swingarm 6 by an arm 11 so as to be positioned in the vicinity of a sideportion of the above-described top ring 4. Due to this arrangement,color CCD camera 10 is able to take an image of a region on thepolishing surface of turntable 2 in the vicinity of top ring 4 during apolishing process. Preferably, color CCD camera 10 may be installed inthis specified position above turntable 2 located downstream withrespect to rotation thereof, where semiconductor wafer W is more likelyto slip out. If color CCD camera 10 is fixedly attached to top ringswing arm 6 in a manner discussed above so as to be swung therewith,with the top ring swing arm 6 serving as a swing motion mechanism fortop ring 4, then even in such a case that polishing is performed whileswinging top ring 4, advantageously an image-taking position of colorCCD camera 10 may be normally fixed with respect to top ring 4. It is amatter of course that color CCD camera 10 may be operatively mounted toother mounting structure, such as an arm independently arrangedseparately from the top ring 4, so that this mounting structure may beswung to position color CCD camera 10 in the vicinity of a side portionof top ring 4.

Image processing section 40 is designed so as to receive image data ofthe polishing surface acquired by color CCD camera 10, to determinewhether or not any foreign matter exists in a region acquired as animage, and then to output this result of determination to apparatusoperation control section 45.

Apparatus operation control section 45 controls an overall operation ofpolishing apparatus 110, and specifically, it provides an independentcontrol of the number of revolutions for turntable 2 and top ring 4, andin addition it also controls a pressure force of semiconductor wafer Wagainst the polishing surface by moving top ring 4 up or down, a swingmotion of top ring swing arm 6 and/or an amount of supply of slurry S.

Next, a detection method of foreign matter on a polishing surface duringa polishing process of a substrate by polishing apparatus 110 will bedescribed in detail.

FIG. 2 is a general flow diagram showing a method for detecting aslip-out event of semiconductor wafer W during a polishing process byusing above-described color CCD camera 10 or the like. Semiconductorwafer W held on the under surface of top ring 4 in a manner as describedabove is brought into contact with the polishing surface of polishingtool 1 and polished by rotating motions of top ring 4 and turntable 2,while during this period, image processing section 40 receives imagesacquired by color CCD camera 10 at a rate of some ten times to somehundred times per second (Step 1), and determines whether or not anyforeign matter, typically a part of semiconductor wafer W which hasslipped out of the top ring 4, exists on the polishing surface based ona specified determination method (Step 2).

Specifically, the determination method includes, for example, thefollowing methods:

(Determination Method 1)

First of all, a color representative of semiconductor wafer W, whichwill be determined as the foreign matter, should have been input andstored in advance as a reference color. Image processing section 40compares a color of each point in the image of the image data receivedfrom color CCD camera 10 with the reference color individually so as toidentify that the color of the point represents the color of the foreignmatter or the color of the polishing surface. Then, in the image at acertain moment, when an area of points (a surface area formed by thepoints) identified to represent the color of the foreign matter hasextended to be ultimately larger than a previously determined specificarea (a threshold value), image processing section 40 determines thatforeign matter exists on the polishing surface.

That is, as shown in FIG. 4 by way of example, in the image data at acertain moment received from color CCD camera 10, if an area of pointsscreened out to be identified as foreign matter (the area painted intoblack) is not larger than a predetermined specific area (the thresholdvalue) as represented by image P1, then image processing section 40determines that there is no foreign matter. On the other hand, if thearea of the points screened out to be identified as the foreign matter(the area painted into black) has extended to be finally larger than thepredetermined specific area (the threshold value) as represented byimage P2, then image processing section 40 determines that the foreignmatter exists on the polishing surface.

If the above-described specific area is set to be small, then detectionsensitivity will be enhanced but there may be a possibility of anerroneous detection due to wrong identification. An optimal set area maybe varied depending on a range of the image acquired by color CCD camera10, a size of semiconductor wafer W, and a relationship betweenfrequency of image processing and a number of revolutions of turntable2, and preferably the set specific area should be around one half of thetotal area of semiconductor wafer W. Further, preferably the referencecolor should be set to have a width of color (a certain range of wavelength) rather than a single color, whereby a more stable screening anddetermination can be provided.

Although in the above-discussed determination method, it is determinedthat foreign matter exists on the polishing surface when the area of thepoints screened out to be identified as the foreign matter has extendedover the predetermined threshold value, the area of the foreign mattershould not be necessarily a criterion, but alternatively imageprocessing section 40 may determine that foreign mater exists when anarea, which has not been screened out as the foreign matter, has beenreduced to be ultimately smaller than a predetermined area (a thresholdvalue).

(Determination Method 2)

The color of the semiconductor wafer representing foreign matter hasbeen set and stored in determination method 1, but determination method2 employs instead a color of the polishing surface to be set and storedas the reference color. In this case also, image processing section 40compares a color of each one of points making up an image of image datareceived from color CCD camera 10 with the reference color individually,and during this comparison, image processing section 40 determines apoint having the color from among the reference color to be foreignmatter.

Then, similarly to determination method 1, in the image at a certainmoment, when an area of the points screened out to be identified asforeign matter has extended to be ultimately larger than a predeterminedspecific area (threshold value), image processing section 40 determinesthat the foreign matter exists on the polishing surface. Preferably, thereference color should be set to cover a certain range of color. An areaof the foreign matter should not necessarily be a criterion, butalternatively image processing section 40 may determine that foreignmatter exists when an area, which has not been screened out as theforeign matter, has been reduced to be ultimately smaller than apredetermined area (the threshold value).

It is to be appreciated that since generally slurry S is being suppliedduring polishing of a substrate and this may change a color of thepolishing surface, in this determination method, the reference colorshould be set also by taking a color of the slurry S (the color of thepolishing surface changed by the supplied slurry S) into consideration.There will be also a case where the color of the polishing surface ischanged when slurry S is replaced by purified water for performing, whatis called, water polishing or when slurry S is changed from one type toanother during polishing, depending on a polishing process applied.

In either case, each of different colors of the polishing surfacegenerated by supply of respective different slurries S should have beenset in advance as the reference color, and in a case of starting,switching or stopping of supply of slurry S under control of apparatusoperation control section 45, apparatus operation control section 45 mayoutput a signal indicative of the operation to image processing section40 so as to switch the reference color from one color to another to beused in determination by image processing section 40 so that imageprocessing section 40 may make a correct determination on whether or notforeign matter exists on a basis of this newly changed reference color.This may enable a stable detection of foreign matter.

(Determination Method 3)

Determination method 3 employs polishing tool 1 having a polishingsurface patterned with two different colors. For example, a color of thepolishing surface of polishing tool 1 may have a color pattern of radiallines in two different colors consisting of bright color areas a1 anddark color areas a2 arranged alternately as shown in FIG. 3( a), or acolor pattern of a check pattern as shown in FIG. 3( b). Either patternis represented by black and white in the drawings, but preferably actualcolors should be chromatic colors. Each element of the above-describedpatterns should be made small enough in comparison with a range of animage taken by color CCD camera 10 so that a ratio of a total areaoccupied by one color in the image acquired during rotation of turntable2 to that occupied by the other color may change little or may beapproximately constant. Alternatively, a pattern formed to be parallelwith a proceeding direction of the polishing surface, or a coaxialcircular pattern for the turntable, may be employed to eliminatesubstantially a change in ratio of one color to the other colorotherwise caused by movement of the polishing surface.

These two colors in the pattern should have been set and stored inadvance as reference colors in image processing section 40, and a totalarea of each one of the reference colors occupying an image at a certainmoment is determined respectively. In determination of whether or notforeign matter exists, it is determined that the foreign matter existswhen either one of areas of two different colors has fallen out of arange of the change in area due to the rotation of turntable 2, to besmaller than a specified area. If a color of the polishing surface issimilar to a color of the foreign matter, there will be a possibilitythat determination of whether or not foreign matter exists is uncertain,but according to this method using two different reference colors,foreign matter of any color would be apparently different from at leasteither one of the two different reference colors, and so detection canbe performed with higher reliability. In this case also, preferably eachreference color should be set to cover a certain range of color.

Three different determination methods have been described as embodimentsof the present invention, and since all three methods according to thepresent invention employ a color camera as an image taking device, eachone of points in an image acquired by the color camera contains anindividual set of gradient data for each one of three primary colors.Owing to this, each of the gradient data may be compared individuallyand thereby a difference in color pertaining to an object can bedetected, which could not have been detected through a comparison ofbrightness in a black and white image or a monotone monochrome image.

Referring again to FIG. 2, when the image processing section 40 hasdetermined that no foreign matter exists according to one of theabove-described determination methods, operation of polishing apparatus110 is continued and the above-discussed determination process (Step 1and Step 2) may be sequentially repeated.

On the contrary, when image processing section 40 has determined thatforeign matter exists according to one of the above-describeddetermination methods, a signal indicative of that determination is sentfrom image processing section 40 to apparatus operation control section45, which in response to this, stops immediately a polishing operationin order to prevent damage to semiconductor wafer W as well as topolishing apparatus 110. (Step 3). Specifically, rotating motions ofturntable 2 and top ring 4 are stopped, and top ring 4 is lifted toseparate from polishing tool 1. Further, any alarm sounds or alarmsignals may be sent to a central control room in a semiconductormanufacturing plant.

It is to be noted that as is the case with a polishing apparatusequipped with the cleaning unit shown in FIG. 5, if the facility ofpolishing apparatus 110 (110 a, 110 b) comprises a plurality ofturntables 2 (2 a, 2 b) and a plurality of top rings 4 (4 a, 4 b),and/or comprises a built-in cleaning unit (cleaning and drying unit126), only operation of the polishing apparatus in concern (for example,apparatus 110 a) may be stopped but polishing apparatus (110 b),cleaning unit 126 and related structure may continue their specifiedoperations.

Although the present invention has been illustrated and described withreference to the preferred embodiments, the present invention is notlimited to those embodiments, and many different variations may be madewithout departing from the scope of the disclosure in the claims and thetechnical concept described in this specification and the attacheddrawings. It is to be noted that other shapes and structures, which arenot directly illustrated in the specification and drawings but canachieve an operation and/or effect of the present invention, areintended to fall within the scope of the technical concept of thepresent invention.

For example, although polishing apparatus 110 using rotary turntable 2has been illustrated in the above embodiments, it is needless to saythat the present invention is applicable to such a polishing apparatushaving a configuration in which a substrate is pressed against apolishing belt moving linearly. Thus, the present invention isapplicable to a polishing apparatus of any configuration so long as itcomprises a polishing surface and a substrate holder device, in which asubstrate held by the substrate holder device is pressed against thepolishing surface, with a surface to be polished of the substratebrought into contact with the polishing surface, and then the substrateand the polishing surface are driven to make a relative movement to eachother so as to polish the substrate.

Although the description in the above embodiment has been directed to anexample for detecting semiconductor wafer W slipped out of top ring 4 asforeign matter, it is needless to say that the present invention isapplicable to detection of a variety of types of foreign matter otherthan the semiconductor wafer.

Although the number of colors used is two in the above determinationmethod 3, three or more colors may be used, and in this case those threeor more colors (or a specified number of colors selected from amongthem) may be used as reference colors for determination of foreignmatter.

EFFECT OF THE INVENTION

According to the present invention, as described above in detail, sincea color camera has been employed as a camera used to take an image of apolishing surface, and each one of points in an acquired image containsa set of color gradient data for each one of three primary colors, whichwill be compared individually, a difference in color pertaining to anobject can be detected more precisely, which could not have beenachieved in a conventional comparison of contrast, and advantageously,existence of any foreign matter can be detected in a more reliablemanner, thus providing a superior effect in ensuring that both of asubstrate and polishing apparatus can be protected from possible damage.

1. A polishing apparatus comprising: a polishing surface; a substrateholder for holding a substrate and pressing a surface of the substrateagainst said polishing surface while said substrate holder and saidpolishing surface move relative to one another so as to polish thesurface of the substrate; a camera for acquiring color image data of aregion of said polishing surface; an image processor for determiningwhether or not foreign matter exists on said polishing surface based ona color condition of the color image data as acquired by said camera,said image processor including (i) identifying means for identifyingwhether or not a color of each point in the color image data as acquiredby said camera is identical with a color of foreign matter, which colorof foreign matter has been previously stored as a reference color, and(ii) determination means for determining existence of foreign matterwhen a total area of points, each of which corresponds to the referencecolor, exceeds a predetermined threshold value.
 2. The polishingapparatus according to claim 1, further comprising: a control unit foroutputting a signal to switch the reference color to another color. 3.The polishing apparatus according to claim 2, further comprising: pluraltypes of polishing liquids to be supplied to said polishing surface,wherein said control unit is also for controlling which of the pluraltypes of polishing liquids is to be supplied to said polishing surface,with said control unit being for outputting the signal to switch thereference color to another color based which of the plural types ofpolishing liquid is to be supplied to said polishing surface.
 4. Thepolishing apparatus according to claim 1, wherein the predeterminedthreshold values depends on a range of the color image data as acquiredby said camera, a size of the substrate, or a relationship betweenfrequency of processing the color image data and a number of revolutionsof said polishing surface, and said reference color has a width ofcolor.
 5. A method for detecting foreign matter on a polishing surface,while polishing a substrate by pressing the substrate against thepolishing surface and moving the substrate and the polishing surfacerelative to one another, said method comprising: acquiring color imagedata of a predetermined region on said polishing surface by using acamera; identifying whether or not a color of each point in said colorimage data acquired by said camera is identical to a color of foreignmatter, which color of foreign matter has been previously stored as areference color; and determining that foreign matter exists (i) when atotal area of points, each of which corresponds to said reference color,exceeds a first predetermined threshold value, or (ii) when a total areaof points, each of which does not correspond to said reference color,decreases to less than a second predetermined threshold value.
 6. Themethod according to claim 5, wherein either of the first or secondpredetermined threshold values depends on a range of said color imagedata acquired by said camera, a size of said substrate, or arelationship between frequency of processing said color image data and anumber of revolutions of said polishing surface, and said referencecolor has a width of color.
 7. A polishing apparatus comprising: apolishing surface; a substrate holder for holding a substrate andpressing a surface of the substrate against said polishing surface whilesaid substrate holder and said polishing surface move relative to oneanother so as to polish the surface of the substrate; a camera foracquiring color image data of a region of said polishing surface; and animage processor for determining whether or not foreign matter exists onsaid polishing surface based on a color condition of the color imagedata as acquired by said camera, said image processor including (i)identifying means for identifying whether or not a color of each pointin the color image data as acquired by said image processor is a colorof foreign matter, and (ii) determination means for determiningexistence of foreign matter when a predetermined threshold value isexceeded by a total area of points, each of which is the color offoreign matter.
 8. A polishing apparatus comprising: a polishingsurface; a substrate holder for holding a substrate and pressing asurface of the substrate against said polishing surface while saidsubstrate holder and said polishing surface move relative to one anotherso as to polish the surface of the substrate; a camera for acquiringcolor image data of a region of said polishing surface; an imageprocessor for determining whether or not foreign matter exists on saidpolishing surface based on a color condition of the color image data asacquired by said camera, said image processor including (i) identifyingmeans for identifying whether or not a color of each point in the colorimage data as acquired by said camera is identical with a color offoreign matter, which color of foreign matter has been previously storedas a reference color, and (ii) determination means for determiningexistence of foreign matter when a total area of points, each of whichdoes not correspond to the reference color, decreases to less than apredetermined threshold value.